This present invention relates generally to glass optical elements, and more particularly to molded glass optical elements with datum(s) formed therein in the molding process that decrease the difficulty of subsequent manufacturing steps.
Various methods and apparatus for the compression molding of glass optical elements are known in the prior art. With these methods and apparatus, optical element preforms sometimes referred to as gobs are compression molded at high temperatures to form glass lens elements. The basic process and apparatus for molding glass elements is taught in a series of patents assigned to Eastman Kodak Company. Such patents are U.S. Pat. No. 3,833,347 to Engle et al., U.S. Pat. No. 4,139,677 to Blair et al., and U.S. Pat. No. 4,168,961 to Blair. These patents disclose a variety of suitable materials for construction of molds used to form the optical surfaces in the molded optical glass elements. Those suitable materials for the construction of the molds include glasslike or vitreous carbon, silicon carbide, silicon nitride, and a mixture of silicon carbide and carbon. In the practice of the process described in such patents, a glass preform or gob is inserted into a mold cavity with the mold being formed out of one of the above mentioned materials. The molds reside within a chamber in which a non-oxidizing atmosphere is maintained during the molding process. The preform is then heat softened by increasing the temperature of the mold to thereby bring the preform up to a viscosity ranging from 107-109 poise for the particular type of glass from which the preform has been made. Pressure is then applied to force the preform to conform to the shape of the mold cavity. The mold and preform are then allowed to cool below the glass transition temperature of the glass. The pressure on the mold is then relieved and the temperature is lowered further so that the finished molded lens can be removed from the mold.
Molded glass lenses may be manufactured with upper and lower molds residing in a cylindrical mold sleeve (U.S. Pat. No. 5,718,850 to Takano et al.). In such a process the final molded lens element is typically cylindrical (and circular in cross-section). The diameter and concentricity of the cylinder are critical to subsequent handling, positioning and mounting operations. Therefore, it has been necessary to control the diameter of the cylinder either during molding, or during a subsequent grinding operation. Controlling the diameter during molding is difficult. Although a cylindrical mold sleeve produces a lens with a well-constrained outer diameter, molding tool life can be decreased due to a variety of factors. One contributor to decreased molding tool life is variability in the volume of the preforms. The preforms are the glass material (usually in the shape of a sphere) from which the lenses are molded. If the preform volume is slightly larger than the mold cavity, the excess glass can exert excessive force on the cylindrical sleeve during molding. It can also become difficult to remove the lenses from the cylindrical sleeve after multiple molding cycles.
Grinding a specified outer diameter on a lens after molding is often referred to as centering. As lens elements become smaller it becomes increasingly difficult to accurately center such lens elements as well as to position and align such elements in subsequent assembly operations.
A lens geometry is needed which allows for accurate centering, handling, positioning, and mounting operations and that does not rely on the accuracy of the outside diameter of the cylindrical body of the lens as molded.
It is therefore an object of the present invention to provide a molded lens having a geometry that allows for accurate centering, handling, positioning, and mounting operations after molding.
It is a further object of the present invention to provide a molded lens having a geometry that does not rely on the accuracy of the outside diameter of the cylindrical body of the lens for post molding operations.
Yet another object of the present invention is to provide a molded lens having a geometry that is not critically dependent upon optical element preform volume for the creation of a reference surface.
Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by providing a molded lens that includes a molded two-dimensional reference surface at a first end of the lens body, a first molded optical surface that is longitudinally offset from the two-dimensional reference surface, and a molded second optical surface at a second end of the lens body. The first and second optical surfaces may be plano, convex or concave. The molded two-dimensional reference surface is an annular plano surface. The lens body (that portion of the lens that is between the second optical surface and the molded two-dimensional reference surface or the molded two-dimensional reference surface, and outside the diameters of the second optical surface and the molded two-dimensional reference surface) may be allowed to partially or fully free-form during molding or may be constrained during molding to provide a generally cylindrical shape to the lens body. If the lens body is allowed to free-form, it is subsequently subjected to a grinding operation to yield a generally cylindrical shape. Whether the generally cylindrical shape of the lens body is accomplished by molding or grinding, the generally cylindrical shape may include an additional datum surface(s) formed therein. Also, the molded lens of the present invention may include a molded three-dimensional reference surface at the second end of the lens body. If the molded lens includes a molded three-dimensional reference surface at the second end of the lens body, that reference surface will be interrupted by the second optical surface. The first and second optical surfaces are designed to image light from an object point to an image point. The molded two-dimensional reference surface is of a specified shape and location with respect to the first and second optical surfaces. By physically locating the lens with the molded two-dimensional reference surface and one of the first or second optical surfaces, the lens can be held in a given orientation. Thus, the molded reference surface(s) at the end(s) of the cylindrical body allow for accurate and safe capture, positioning, handling, and placement for subsequent finishing operations, allowing for the creation of one or more additional lens datums. These finishing operations can include, but are not limited to, grinding, polishing and cutting. These functions of capture, positioning, handling, and placement for subsequent operations can be performed using a centering cup that engages the molded reference surface(s) at the end(s) of the cylindrical body thereby allowing subsequent operations to be performed without reliance on the outside diameter of the lens body.
As mentioned above, the first molded optical surface is longitudinally offset from the two-dimensional reference surface. That is, the first molded optical surface is positioned along the cylindrical or optical axis of the lens but the two-dimensional reference surface and the first molded optical surface reside at different distances from the second optical surface. The offset may be such that the first molded optical surface is closer to or further from the second optical surface as compared to the molded two-dimensional reference surface. In other words, the offset may take the form of an axial recess or an axial projection. In fact, the offset may be simultaneously a partial axial recess and a partial axial projection.
The lens of the present invention can be made with an angled plano optical surface, a convex optical surface and a lens datum. This lens datum can then be used for subsequent processing operations (such as grinding) to add additional datums to the lens. These additional datums can be placed in a precise location with respect to the optical axis of the lens element. One of the additional lens datums can be a cylindrical surface that enables mounting of the lens either in a V-groove type structure or in a precise tube. In addition, the lens can be molded without the need for a precisely controlled cylindrical preform. The lens can be centered using existing centering equipment. Because the lens does not have to be held on the optical plano surface, it reduces the chances of scratching this surface. Scratches can lead to scatter and reduce the light transmitted by the lens. This is particularly important for situations where the beam diameter of the light directed onto the plano optical face may be only 50 microns such as those optical elements used in conjunction with optical fibers. In such an instance, a scratch of only a few microns in size could cause a measurable decrease in the amount of light transmitted by the lens to the receiving fiber.
Either as a result of a post-molding grinding operation or as result of the molding operation itself, the lenses of the present invention typically will be xe2x80x9cgenerally cylindrical.xe2x80x9d Further, such lenses will typically be circular in cross-section. However, there may be lens applications where it is beneficial to form generally cylindrical lenses which, in cross-section perpendicular to lens axis, are not circular (e.g. elliptical). Thus, the term xe2x80x9cgenerally cylindricalxe2x80x9d as used herein is intended to include cylindrical lenses that may or may not be circular in cross-section. In addition, the term xe2x80x9cgenerally cylindricalxe2x80x9d as used herein is intended to include those lenses which have datum(s) formed in the cylindrical surface thereof such as, for example, flat datum(s) and recessed datum(s) as will be discussed in more detail hereafter. Therefore, the formation of such datum(s) in the cylindrical surface of a lens will not remove such lens from the definition of xe2x80x9cgenerally cylindricalxe2x80x9d as used herein.